Light emitting devices, such as light emitting diodes (LEDs) and laser diodes (LDs), which use a Group III-V or Group II-VI compound semiconductor material, may render various colors such as red, green, blue, and ultraviolet by virtue of development of thin film growth technologies and device materials. It may also be possible to produce white light at high efficiency using fluorescent materials or through color mixing. Further, the light emitting devices have advantages, such as low power consumption, semi-permanent lifespan, fast response time, safety, and environmental friendliness as compared to conventional light sources, such as fluorescent lamps and incandescent lamps.
Therefore, these light emitting elements are increasingly applied to transmission modules of optical communication units, light emitting diode backlights as a replacement for cold cathode fluorescent lamps (CCFLs) constituting backlights of liquid crystal display (LCD) devices, lighting apparatuses using white light emitting diodes as a replacement for fluorescent lamps or incandescent lamps, headlights for vehicles and traffic lights.
Nitride semiconductor light emitting devices employ a sapphire substrate, which is an insulating substrate, because there is no commercially-available substrate having the same crystalline structure as a nitride semiconductor material such as GaN while being lattice matchable with the nitride semiconductor material. In such a nitride semiconductor light emitting device, there may be lattice constant and thermal expansion coefficient differences between the sapphire substrate and a GaN layer grown over the sapphire substrate. As a result, lattice mismatch may occur between the sapphire substrate and the GaN layer, so that numerous crystal defects may be present in the GaN layer.
Such crystal defects may cause an increase in leakage current. When external static electricity is applied to the light emitting device, the active layer of the light emitting device, which has numerous crystal defects, may be damaged by an intense field generated due to the static electricity. Generally, it is known that there are crystal defects (threading defects) in a GaN thin film at a density of 1010 to 1012/cm2.
Such a nitride semiconductor light emitting device, which has numerous crystal defects, may be very weak against electrical impact. To this end, technologies and standardization for protection of nitride semiconductor light emitting devices from static electricity and lightning are being highlighted as very important technical issues.
Generally, a conventional GaN light emitting device has electrostatic discharge (ESD) characteristics that the device can withstand, in a human body mode (HBM), static electricity of up to several thousand volts in a forward direction, but cannot withstand static electricity of several hundred volts in a reverse direction. Such ESD characteristics are exhibited mainly due to the crystal detects of the device, as mentioned above.
In order to improve such ESD characteristics, and thus to protect the light emitting device from ESD, a proposal to connect a Schottky diode or a Zener diode with the light emitting device in parallel has been made. However, such a proposal causes troublesomeness and an increase in manufacturing costs in that a Schottky diode or a Zener diode is separately required.
Therefore, it is necessary to improve the structure of the light emitting device, and thus to improve the ESD characteristics of the light emitting device.